ARTICLE IN PRESS
VOLGEO-04202; No of Pages 11
Journal of Volcanology and Geothermal Research xxx (2009) xxx–xxx
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Journal of Volcanology and Geothermal Research
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / j v o l g e o r e s
Thermochemical evolution of young rhyolites at Yellowstone: Evidence for a cooling
but periodically replenished postcaldera magma reservoir
Jorge A. Vazquez a,⁎, Stephanie F. Kyriazis a, Mary R. Reid b, Robin C. Sehler a, Frank C. Ramos c
a
b
c
Department of Geological Sciences, California State University, Northridge, 18111 Nordhoff Street, Northridge, California, 91330, USA
Department of Geology, Northern Arizona University, Flagstaff, Arizona, 86011, USA
Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico, 88003, USA
a r t i c l e
i n f o
Article history:
Received 14 April 2008
Accepted 24 November 2008
Available online xxxx
Keywords:
Yellowstone
rhyolite
geothermometry
QUILF
TitaniQ
a b s t r a c t
Between circa 170 ka and 75 ka, more than 600 km3 of high-silica rhyolite composing the Central Plateau
Member (CPM) lavas of the Plateau Rhyolite erupted within the margins of Yellowstone caldera. The
compositions of CPM ferromagnesian phenocrysts and groundmass glasses provide important constraints on
the thermochemical evolution of the youngest postcaldera rhyolites erupted within the Yellowtone caldera.
Phenocryst and groundmass glass compositions are generally correlated with eruption age, with increasing
Fe/Mg in ferromagnesian silicates, decreasing Ti in quartz, and increasing concentrations of incompatible
elements erupted in melts over time. Crystallization and eruption temperatures from Fe–Ti oxide, QUILF, and
TitaniQ geothermometry reveal that CPM rhyolites evolved between approximately 750 °C and 900 °C, with a
general trend to cooler magma compositions over time. Titanium zoning within single quartz phenocrysts
suggests that magma temperature fluctuated over tens of degrees during crystallization. The Pb–Nd isotope
composition of CPM glasses indicates that as the rhyolites evolved to lower temperatures, they became morejuvenile in isotopic composition. These age-correlated compositional characteristics and the results from
geothermometry suggest open-system evolution of a cooling magma reservoir that was periodically
replenished with new rhyolite. Petrographic and isotopic differences between CPM rhyolites and older
postcaldera rhyolites suggest that wallrock remelting was most important during the early postcaldera
history of Yellowstone.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
The Yellowstone hotspot is responsible for generating many of the
largest eruptions of rhyolite on Earth. These eruptions have produced
extensive rhyolitic ignimbrites and lavas that cover large areas of the
Snake River Plain (SRP) and Yellowstone Plateau (e.g., Bonnichsen et al.,
2008) as well as regionally extensive beds of ash tuff (Perkins and Nash,
2002). In some cases, this rhyolitic volcanism has formed large
calderas, whereas in others it has formed volcanic centers with vents
distributed over hundreds of km2 (Bonnichsen et al., 2008). Field and
petrologic studies (e.g., Hildreth et al., 1984, 1991; Christiansen, 2001;
Hughes and McCurry, 2002; Bonnichsen et al., 2008) indicate that SRP
and Yellowstone rhyolites are tapped from voluminous reservoirs that
are ultimately fed and sustained by intrusions of hotspot basalt. These
rhyolites form distinct “composition and time” groups with coherent
geochemical traits reflecting magma evolution over timescales of
hundreds of thousand of years (Bonnichsen et al., 2008). The youngest
silicic volcanism of the hotspot, located on the Yellowstone Plateau, has
⁎ Corresponding author. Tel.: +1 818 677 4670.
E-mail address: [email protected] (J.A. Vazquez).
formed three calderas over the last 2 Ma (Christiansen, 1984, 2001).
Despite the volumetric significance of these eruptions, the chronology
of thermal and chemical changes associated with the evolution of
Yellowstone rhyolites is unclear. The relative roles of fractional
crystallization, hybridization, and remelting of crust are poorly
constrained, and the conclusions of recent studies are seemingly at
odds with each other (e.g., Hildreth et al., 1984, 1991; Bindeman and
Valley, 2001; Vazquez and Reid, 2002; Boroughs et al., 2005; Bindeman
et al., 2007, 2008; Leeman et al., 2008).
To understand the magmatic evolution of the youngest postcaldera
rhyolites at Yellowstone caldera, we have linked geothermometry with
the compositional variability of ferromagnesian phenocrysts and
groundmass glasses from lava flows composing the postcaldera Central
Plateau Member (CPM) of the Plateau Rhyolite. In addition, we compare
the CPM rhyolites to one of the youngest lavas (Scaup Lake flow) from
the next oldest group of postcaldera rhyolites. CPM rhyolites represent
snapshots into the evolution of Yellowstone's postcaldera magma
reservoir because they erupted sequentially over an ~100 thousand
year interval (Christiansen, 2001; Christiansen et al., 2007). Two models
have been proposed for Yellowstone's postcaldera evolution: 1) opensystem differentiation of a voluminous reservoir that underwent
fractionation, assimilation, and recharge (Hildreth et al., 1984, 1991;
0377-0273/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jvolgeores.2008.11.030
Please cite this article as: Vazquez, J.A., et al., Thermochemical evolution of young rhyolites at Yellowstone: Evidence for a cooling but
periodically replenished postcaldera..., Journal of Volcanology and Geothermal Research (2009), doi:10.1016/j.jvolgeores.2008.11.030
ARTICLE IN PRESS
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J.A. Vazquez et al. / Journal of Volcanology and Geothermal Research xxx (2009) xxx–xxx
Vazquez and Reid, 2002), and 2) localized small-volume remelting of
shallow caldera wallrocks by intrusions of hot basalt (Bindeman and
Valley, 2001; Bindeman et al., 2008). Our results indicate that CPM
minerals and glasses have a general affinity to each other and form agecompositional-temperature trends that are consistent with cooling and
fractionation of a reservoir undergoing recharge, assimilation, and
mixing. Wholesale remelting and crystal recycling were most important
in the early portion of the postcaldera history.
2. Postcaldera volcanism and Central Plateau Member rhyolites
Members of the Plateau Rhyolite, including the CPM, erupted
during the youngest of three caldera cycles on the Yellowstone Plateau
(Christiansen, 2001). The third cycle culminated with the eruption of
≥1000 km3 (dense rock equivalent) of compositionally zoned rhyolite
and the collapse of Yellowstone caldera (Christiansen, 1984, 2001) at
639 ± 2 ka (Lanphere et al., 2002). This caldera-forming rhyolite forms
the widespread Lava Creek Tuff (Christiansen and Blank, 1972). Since
collapse, Yellowstone caldera has been filled by eruptions of
intracaldera rhyolites. At the same time, extracaldera rhyolites and
basalts erupted from vents north and south of the caldera margins.
Postcaldera volcanism at Yellowstone caldera occurred during two
major phases and primarily generated lavas with a minority of tuffs
(Christiansen, 2001; Christiansen et al., 2007). The oldest phase
erupted low- to high-silica rhyolites (72–76 wt.% SiO2) between 516±7 ka
and 479±10 ka (Gansecki et al., 1996) from the ring-fracture zone of the
caldera (Christiansen, 2001). These early postcaldera rhyolites compose
the Upper Basin Member (UBM) of the Plateau Rhyolite and are well
known for their distinct low δ18O values (Hildreth et al., 1984, 1991;
Bindeman and Valley, 2001; Bindeman et al., 2008). Two UBM rhyolites,
the Scaup Lake and South Biscuit Basin flows, are relatively young with
40
Ar/39Ar eruption ages of ca. 260 ka (Christiansen et al., 2007; Bindeman
et al., 2008).
The youngest phase of postcaldera volcanism formed the CPM
rhyolites (Christiansen and Blank, 1972). CPM rhyolites erupted during
five distinct volcanic episodes with eruption ages clustered at ca. 170 ka,
150 ka, 115 ka, 100 ka, and 75 ka, and with vents aligned along two NWtrending lineaments (Christiansen, 2001; Christiansen et al., 2007).
Between 10–150 km3 of rhyolite were erupted during each episode
(Christiansen et al., 2007). The eruptions generated at least 17 lavas and
two ignimbrites that bury most of Yellowstone caldera's floor and
western rim (Fig. 1). Volumes of individual lavas average ~10 km3, but
are in several cases up to 50–70 km3 (Christiansen et al., 2007).
Resurgence of the caldera floor occurred just after eruption of the
Mallard Lake flow (Christiansen, 2001), which is considered to be a
separate member of the Plateau Rhyolite because it erupted prior to
resurgence (Christiansen and Blank, 1972). The Mallard Lake flow is
nonetheless part of the ~170 ka volcanic episode and directly related to
CPM rhyolites (Christiansen et al., 2007). These youngest postcaldera
rhyolites are more-evolved and have higher δ18O values than the UBM
lavas (Hildreth et al., 1984; Bindeman and Valley, 2001; Christiansen,
2001), and typically contain sanidine, quartz, clinopyroxene, with
fayalite in the youngest lavas and orthopyroxene in the oldest lavas
(Hildreth et al., 1984). A near-absence of hydrous phases and a paucity of
explosive eruptions suggests that CPM eruptions tapped relatively H2Opoor rhyolite (Leeman and Phelps,1981; Hildreth et al.,1984; Balsley and
Gregory, 1998; Christiansen, 2001).
3. Materials and methods
The compositions of minerals and groundmass glasses from
vitrophyric lavas erupted during the five CPM eruptive intervals were
Fig. 1. Distribution of Plateau Rhyolite lavas and tuffs at Yellowstone caldera. Caldera margin is marked by dashed line. CPM rhyolites (grey) erupted from vents (stars) aligned along
two NW-trending lineaments (Christiansen, 2001). Early postcaldera rhyolites (single stipple) of the Upper Basin Member are concentrated in the eastern portion of caldera, with
scattered outcrops along the western vent lineament, including the 257 ± 13 ka Scaup Lake flow (SCL). The Mallard Lake flow (ML, double stipple) is a distinct member although
closely related in age and composition to CPM rhyolites. Extracaldera rhyolites (vertical lines) are concentrated north of the caldera. CPM lavas mentioned in text are labeled. PP:
Pitchstone Plateau; GP: Grants Pass; SC: Spring Creek; DC: Dry Creek; SP: Solfatara Plateau; HV: Hayden Valley, ML: Mallard Lake. Inset shows location of caldera in western U.S.A.
Modified from Christiansen (2001) and Christiansen et al. (2007).
Please cite this article as: Vazquez, J.A., et al., Thermochemical evolution of young rhyolites at Yellowstone: Evidence for a cooling but
periodically replenished postcaldera..., Journal of Volcanology and Geothermal Research (2009), doi:10.1016/j.jvolgeores.2008.11.030
ARTICLE IN PRESS
J.A. Vazquez et al. / Journal of Volcanology and Geothermal Research xxx (2009) xxx–xxx
analyzed. For comparison, phenocrysts and glass from the UBM Scaup
Lake flow were analyzed. The CPM vitrophyres contain 10–30%
phenocrysts of sanidine, quartz, Fe-rich clinopyroxene, and Fe–Ti oxides.
In addition, fayalite occurs in lavas erupted after ca. 130 ka, and
plagioclase and orthopyroxene occur in lavas erupted before ca. 150 ka.
In some cases, these orthopyroxene are overgrown by clinopyroxene.
Accessory zircon, chevkinite, and apatite are ubiquitous and occur as
inclusions in ferromagnesian minerals and groundmass microphenocrysts. Groundmass glasses typically contain microlites of clinopyroxene, Fe–Ti oxides, zircon, chevkinite, and apatite. The Scaup Lake flow
has a mineral assemblage like the oldest CPM rhyolites, but includes a
greater amount of plagioclase and orthopyroxene.
Groundmass glasses were separated for analysis using standard
heavy liquid techniques and hand-picked to ensure purity and
exclusion of phenocrysts. Major, minor, and trace elements in glasses
were measured by X-ray fluorescence and inductively-coupled mass
3
spectrometry at Washington State University. Aliquots of glass
separates were dissolved and doped with NBS 997 Tl for Pb isotope
analysis following Wolff and Ramos (2003). Pb isotope compositions
were measured using a ThermoNeptune MC-ICPMS at University of
California, Santa Cruz using seven collectors in static mode. To correct
for instrumental fractionation, measured Pb isotope ratios were
normalized to NBS 997 Tl isotope composition (205Tl/203Tl = 0.41885).
Analytical details are given in the caption of Table 1.
Mineral compositions were measured using CAMECA SX-100
(Oregon State University) and JEOL 8200 (University of California, Los
Angeles) electron microprobes with 5 nA beams focused to 1–5 mm
spots. Quartz and sanidine were imaged by cathodoluminescence
using a Quanta 600 scanning electron microscope at California State
University, Northridge. Imaged quartz crystals were analyzed for Ti
with a CAMECA ims 6f ion microprobe at Arizona State University using
a 1–2 nA beam of 16O− and a 75 eV offset for energy filtering (e.g.,
Table 1
Major, minor, trace element, and Pb isotope compositions of CPM groundmass glasses
Sample Lava
flow
40
YCV12 Pitchstone
Plateau
Ar/39Ar age 79 ± 10
(ka)
Weight%
75.52
SiO2
0.122
TiO2
11.72
Al2O3
FeO
1.61
MnO
0.039
MgO
0.02
CaO
0.44
3.55
Na2O
5.12
K2O
0.007
P2O5
Sum
98.14
LOI (%)
0.56
Trace elements (ppm)
La
88.45
Ce
171.37
Pr
19.56
Nd
70.23
Sm
15.11
Eu
0.46
Gd
14.22
Tb
2.52
Dy
15.53
Ho
3.14
Er
8.54
Tm
1.25
Yb
7.68
Lu
1.13
Ba
48
Th
27.14
Nb
55.23
Y
82.82
Hf
9.81
Ta
3.95
U
6.35
Pb
32.50
Rb
204.8
Cs
3.96
Sr
3
Sc
1.5
Zr
258
206
204
Pb/ Pb 17.525 ± 0.001
207
Pb/204Pb 15.575 ± 0.001
208
Pb/204Pb 38.243±0.002
143
Nd/144Nd 0.512257±8
YCV06 Grants
Pass
YCV04 Solfatara
Plateau
YCV05 Hayden
Valley
YCV09 West
Yellowstone
YCV07 Spring
Creek
YCV14 Dry
Creek
YCV15 Mallard
Lake
YCV08 Scaup
Lake
72 ± 3
103 ± 8
102 ± 4
114 ± 1
145
166 ± 9
164 ± 14
257 ± 13
75.70
0.134
11.69
1.55
0.037
0.03
0.46
3.51
5.13
0.008
98.25
0.35
75.44
0.234
11.68
2.58
0.061
0.02
0.68
3.64
5.04
0.014
99.40
0.29
76.38
0.142
11.86
1.65
0.040
0.03
0.47
3.61
5.15
0.011
99.35
0.69
75.70
0.133
11.75
1.76
0.040
0.02
0.45
3.56
5.13
0.007
98.54
0.35
77.02
0.139
11.99
1.46
0.036
0.04
0.46
3.53
5.24
0.009
99.93
0.20
77.06
0.155
12.04
1.49
0.035
0.05
0.48
3.43
5.37
0.009
100.11
0.45
75.73
0.149
11.91
1.42
0.034
0.04
0.44
3.35
5.30
0.009
98.39
0.67
73.24
0.160
11.68
1.22
0.028
0.07
0.51
3.07
5.23
0.014
95.22
3.15
92.30
176.59
19.92
71.04
14.65
0.59
13.41
2.31
14.17
2.84
7.65
1.12
6.92
1.04
114
26.83
42.28
73.88
9.12
3.11
6.05
30.48
187.7
3.74
6
1.9
246
17.560 ± 0.001
15.580 ± 0.001
38.272±0.002
n.m.
93.68
182.78
21.27
80.23
16.81
1.81
15.69
2.65
16.01
3.19
8.55
1.23
7.53
1.15
595
20.47
55.79
80.24
14.53
3.62
4.43
27.25
141.7
2.48
9
1.8
488
17.356 ± 0.001
15.546 ± 0.001
38.106±0.002
0.512252±8
95.56
183.19
20.75
74.44
15.71
0.51
14.35
2.49
15.31
3.08
8.41
1.23
7.55
1.12
61
27.34
58.96
80.40
9.94
4.20
6.20
31.16
192.4
4.04
7
1.8
268
17.543 ± 0.001
15.576 ± 0.001
38.252±0.004
0.512261±9
96.23
184.48
20.85
74.81
15.42
0.62
14.01
2.41
14.90
2.94
7.97
1.18
7.10
1.07
112
26.73
56.14
77.39
9.70
3.94
5.90
30.20
188.5
3.82
5
1.9
272
17.548 ± 0.001
15.579 ± 0.001
38.261±0.002
0.512271±9
85.66
163.80
18.24
64.22
13.08
0.51
12.05
2.09
12.78
2.60
7.08
1.04
6.50
0.96
97
27.01
42.92
68.23
8.20
3.18
6.11
29.88
192.6
4.06
6
2.3
213
17.567 ± 0.001
15.582 ± 0.001
38.279±0.002
0.512245±9
86.35
163.83
18.16
63.69
12.78
0.65
11.49
1.96
12.08
2.45
6.66
0.98
6.11
0.90
180
26.50
42.11
64.37
8.00
3.12
5.87
30.00
188.8
3.96
9
2.6
215
17.572 ± 0.001
15.583 ± 0.001
38.279±0.002
0.512246±9
85.92
163.82
18.02
62.86
12.57
0.59
11.34
1.95
11.77
2.42
6.53
0.96
5.96
0.89
160
27.41
48.62
62.09
8.17
3.60
6.01
29.80
205.5
4.00
8
2.6
216
17.573 ± 0.001
15.582 ± 0.001
38.277±0.002
0.512241±7
76.34
145.06
16.02
55.72
11.11
0.96
9.86
1.72
10.57
2.14
5.91
0.88
5.44
0.81
519
25.93
38.09
57.31
6.39
2.99
5.71
27.56
190.5
4.04
27
2.6
172
17.593 ± 0.001
15.590 ± 0.001
38.278±0.001
0.512220±9
Major–minor-trace element analyses performed by XRF and ICPMS at Washington State University GeoAnalytical Laboratory. Eruption ages are from Christiansen et al. (2007). Age of
Dry Creek flow is from dated volcanic stratigraphy. Pb isotope analyses performed on a ThermoNeptune multi-collector ICPMS. During analyses, repeated analysis (n = 6) of NIST 981
(12.5 ppb) standard yielded weighted averages of 206Pb/204Pb: 16.929 ± 0.001, 207Pb/204Pb: 15.482 ± 0.001, 208Pb/204Pb: 36.669 ± 0.002. Nd-isotope compositions from Vazquez and
Reid (2002), except for YCV08, which was measured in static mode on a Sector 54 multi-collector mass spectrometer at Central Washington University. Not measured = n.m. LOI = loss
on ignition.
Please cite this article as: Vazquez, J.A., et al., Thermochemical evolution of young rhyolites at Yellowstone: Evidence for a cooling but
periodically replenished postcaldera..., Journal of Volcanology and Geothermal Research (2009), doi:10.1016/j.jvolgeores.2008.11.030
ARTICLE IN PRESS
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J.A. Vazquez et al. / Journal of Volcanology and Geothermal Research xxx (2009) xxx–xxx
Fig. 2. Correlation between groundmass glass composition and 40Ar/39Ar eruption age from Christiansen et al. (2007). CPM lavas shown as filled circles (open circle: Solfatara Plateau
flow). UBM Scaup Lake flow (open square) erupted ~100,000 years before CPM volcanism. Uncertainties (2σ) on Pb isotope values are smaller than symbols.
Hervig et al., 2006). For analyses, intensities of 48Ti+ and 30Si+
secondary ions generated during sample sputtering were measured
in 20 cycles with a single electron multiplier. Abundances of Ti in
quartz were calculated from 48Ti+/30Si+ yields relative to the mean of
multiple analyses of NIST 610 glass standard. Reproducibility of the
NIST 610 standard was ~1%. Uncertainties (1σ) on Ti concentrations are
approximately ±1–2 ppm.
4. Results
4.1. Trace element and Pb isotope compositions of CPM glasses
CPM groundmass glasses are high-silica rhyolite in composition
and contain trace element concentrations that generally correlate
with eruption age (Table 1). Younger CPM rhyolites contain the
highest concentrations of Fe, Mn, Na, Hf, Ce, Nb, Zr, Y, Pb, and lowest
concentrations of Ti, Al, Mg, K, Sr, Ba, Eu, and Sc (Fig. 2). Concentrations
of elements that typically behave compatibly in rhyolitic magmas,
such as Sr, Ba, Ca, and Ti, generally decrease with decreasing eruption
age (Fig. 2). Two notable exceptions are the Solfatara Plateau and
Grants Pass flows. The Grants Pass flow contains higher Ba, Sr, and Ti,
than coeval Pitchstone Plateau flow, while the Solfatara Plateau flow
has elevated Ba, Zr, Fe, and Ti relative to other CPM lavas. Scaup Lake
(UBM) flow glass contains higher Ba, Sr, and Eu concentrations than
CPM rhyolites. CPM glasses contain variable Pb isotope compositions
that correlate with eruption age, with less radiogenic Pb values associated
with younger rhyolites (Fig. 2). In general, Pb isotope composition
correlates inversely with 143Nd/144Nd composition (Table 1). The Solfatara
Plateau flow is distinct with a significantly less radiogenic composition
than other CPM rhyolites (Fig. 2) including the coeval Hayden Valley flow
(Table 1).
4.2. Composition of ferromagnesian phases and quartz
Representative analyses of the ferromagnesian phases are given
in Appendix A. Compositions of CPM clinopyroxenes are variable
(En03–34Wo34–46) and generally correlate with eruption age (Table 2,
Fig. 3). Clinopyroxene phenocrysts from younger CPM rhyolites have
higher average Fe/Mg than those in older lavas (Fig. 3). Most of these
clinopyroxene phenocrysts are subhedral to euhedral and homogenous in composition (Figs. 3 and 4, Table 2). However, clinopyroxene phenocrysts from at least two rhyolites (Solfatara Plateau and
Mallard Lake flows) contain rims with significantly higher Fe/Mg
than cores (Fig. 3). Fayalitic olivines (Fo01–04) are subhedral to anhedral
and homogenous in composition. Fe concentrations for CPM fayalites
Table 2
Compositions of clinopyroxene and olivine phenocrysts
Rhyolite flow
CPX Mg#
Rims
Cores
Rims
Cores
Pitchstone Plateau
Grants Pass
Solfatara Plateau
West Yellowstone
Summit Lake
Spring Creek
Dry Creek
Mallard Lake
Scaup Lake
16 ± 1
25 ± 1
10 ± 6
22 ± 1
22 ± 1
35 ± 3
42 ± 3
41 ± 1
59 ± 2
16 ± 1
24 ± 2
20 ± 3
22 ± 2
20 ± 2
35 ± 4
44 ± 5
46 ± 2
49 ± 5
5.0 ± 0.1
5.0 ± 0.1
–
2.35 ± 0.03
7.1 ± 0.2
–
–
–
–
–
Olivine Fo#
2.27 ± 0.01
7.0 ± 0.1
Age
(ka)
79 ± 10
72 ± 3
103 ± 8
114 ± 1
124 ± 10
145
166 ± 9
164 ± 14
257 ± 13
Clinopyroxene Mg# (100⁎Mg/(Mg + Fe) atomic) and olivine forsterite (Fo) component
measured by electron microprobe. Averages (± 1 s.d.) represent multiple analyses of 5–
15 individual crystals/flow. Dash indicates olivine is not present or observed. Eruption
ages from Christiansen et al. (2007).
Please cite this article as: Vazquez, J.A., et al., Thermochemical evolution of young rhyolites at Yellowstone: Evidence for a cooling but
periodically replenished postcaldera..., Journal of Volcanology and Geothermal Research (2009), doi:10.1016/j.jvolgeores.2008.11.030
ARTICLE IN PRESS
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Fig. 3. Age-correlated compositions of CPM and Scaup Lake (UBM) clinopyroxenes
(hexagons) and fayalite (squares). Cores denoted by shaded symbols and rims denoted
by open symbols. Labels for different lavas are the same as in Fig. 1.
are distinct between lava flows (Fig. 3), with the highest concentrations
associated with the Solfatara Plateau flow. The Scaup Lake flow
contains both clinopyroxene (En11–35Wo30–52) and orthopyroxene
(En31–48Wo03–08) phenocrysts. These pyroxenes are distinct because
a subset contains cores with exsolved pigeonite and Fe–Ti oxides (Fig. 4).
Quartz occurs as subhedral to anhedral phenocrysts in CPM
rhyolites and the Scaup Lake flow. Compositional zoning in individual
quartz phenocrysts is apparent in cathodoluminescence (CL) images
(Fig. 5). In most of the lavas, quartz phenocrysts contain CL-bright
cores that are rounded and overgrown by distinct rims. Boundaries
between zones in both rims and cores are typically irregular and
truncate zoning. Concentrations of Ti in CPM quartz vary between 65
and 130 ppm (Appendix B) with significant core-to-rim variations
within single crystals (Fig. 7). The highest Ti concentrations for CPM
quartz are associated with Dry Creek flow, whereas the lowest are
associated with the Pitchstone Plateau flow. Quartz phenocrysts in
the Scaup Lake flow have Ti concentrations similar to those from the
Dry Creek flow, but the sense of core-to-rim zoning is the opposite
(Fig. 6). As documented for other igneous quartz (e.g., Muller et al.,
2002; Wiebe et al., 2007; Wark et al., 2007), Ti concentrations
correlate positively with CL brightness (Fig. 7). Quartz phenocrysts
from Pitchstone Plateau, West Yellowstone, Summit Lake, Spring
Creek, and Dry Creek flows contain rims characterized by lower Ti
concentrations and CL brightness than their cores (Figs. 5 and 7). In
contrast, quartz phenocrysts from Scaup Lake and Solfatara Plateau
flows have rims with higher CL brightness and Ti concentrations than
cores (Fig. 5).
4.3. Geothermometry of ferromagnesian phases
Temperatures using the ferromagnesian phases were determined
using the QUILF algorithm of Anderson et al. (1993). QUILF uses the
compositions of coexisting pyroxenes, olivine, quartz, and Fe–Ti
oxides, or pairs of these phases, to calculate crystallization temperature, pressure, and oxygen fugacity, and to assess equilibrium between
these minerals. For QUILF calculations, rim and/or core compositions
of coexisting or intergrown clinopyroxene and fayalite are used, as
well as ilmenite and magnetite pairs that are free of exsolution and
pass the Mn/Mg equilibrium test of Bacon and Hirschmann (1988). In
addition, the observed forsterite component of the fayalite is allowed
to vary in the calculations because the rims of anhedral phenocrysts
might not be in equilibrium with the rims of coexisting euhedral
clinopyroxene. A pressure of 100 MPa is assumed for each calculation,
which is supported by the minimum melt composition of CPM
rhyolites (Doe et al., 1982).
5
QUILF solutions utilizing Fe–Ti oxides, clinopyroxene and fayalite yield
high residuals. High residuals for solutions to multi-phase assemblages
indicate disequilibrium, usually due to differential rates of re-equilibration
between silicates and oxides or the presence of xenocrysts (cf. Anderson
et al., 1993). Accordingly, temperatures were determined for separate
clinopyroxene-fayalite (e.g., Ren et al., 2006) and Fe–Ti oxide (e.g., Frost
and Lindsley, 1992) pairs. Observed compositions for coexisting clinopyroxene and fayalite are essentially identical to those calculated for
equilibrium pairs (Table 3). Resulting clinopyroxene-fayalite temperatures
range from 755 °C to 845 °C (Table 3) with the lowest temperatures
associated with the Pitchstone Plateau flow (Fig. 6). Increasing the
assumed pressure to 400 MPa, which corresponds to the greatest depth
(~12 km) of the seismic low velocity zone beneath present-day Yellowstone (Husen et al., 2004), yields clinopyroxene-fayalite temperatures that
are systematically ~30° higher. Fe–Ti oxide pairs yield temperatures
between 800°–900 °C, and indicate fO2 at or within ~1 log unit of the
quartz–fayalite-magnetite buffer (Table 3). As with the clinopyroxenefayalite thermometry, the Pitchstone Plateau flow yields the lowest Fe–Ti
oxide temperature, whereas the Dry Creek flow yields the highest (Fig. 6).
Orthopyroxene–clinopyroxene pairs from the Scaup Lake flow yield a
temperature of ~900 °C.
4.4. Quartz geothermometry
The Ti concentrations of quartz phenocrysts were used to calculate
temperatures using the TitaniQ thermometer of Wark and Watson (2006).
Application of the TitaniQ thermometer to rutile-absent magmas requires
an assumption or knowledge of melt TiO2 activity (aTiO2) during quartz
crystallization (Wark and Watson, 2006). Following the method of Wark
et al. (2007), the compositions and temperatures from coexisting Fe–Ti
oxides are used to calculate aTiO2 for individual rhyolites. Calculated aTiO2
ranges between 0.3 and 0.6 (Table 3). Using their respective aTiO2 values,
quartz phenocrysts from the CPM and Scaup Lake flow rhyolites yield
temperatures between approximately 750 °C to 900 °C (Table 3).
Assuming constant aTiO2 during crystallization, the variations of Ti
concentration within single quartz phenocrysts indicate temperature
fluctuations of up to 40° (Fig. 7). Except for the Solfatara Plateau flow, rims
on CPM quartz phenocrysts yield temperatures that are mostly 20–40°
lower than their cores, and both core and rim temperatures decrease with
eruption age (Fig. 6). In contrast, high Ti rims on quartz phenocrysts from
the Solfatara Plateau and Scaup Lake flows yield temperatures that are 10–
20° (up to 40°) higher than their cores (Fig. 6).
5. Discussion
Results from the compositional analyses and thermometry of CPM
phenocrysts and glasses include the following: 1) compositions of
sequentially erupted melts and ferromagnesian silicates generally
correlate with 40Ar/39Ar eruption age such that the youngest rhyolites
are associated with the most-evolved compositions, 2) the oldest CPM
rhyolites yield the highest temperatures using multiple geothermometers, whereas the youngest and most-voluminous rhyolite
(Pitchstone Plateau) yields the lowest temperature, 3) the Pb isotope
compositions of CPM melts become increasingly juvenile with time,
and 4) mineral compositions are distinct between eruptive episodes
and texturally distinct from UBM rhyolites. These observations
provide important insights into the thermochemical evolution of
CPM rhyolites and the origin of postcaldera rhyolites at Yellowstone
caldera.
5.1. Phenocryst and glass record of evolving magma compositions
The compositional variation of CPM phenocrysts and glasses confirm
the compositional trends originally documented by Christiansen (1984)
and Hildreth et al. (1984) as well as the affinity between CPM crystals
and melts (Leeman and Phelps, 1981). The general correlation between
Please cite this article as: Vazquez, J.A., et al., Thermochemical evolution of young rhyolites at Yellowstone: Evidence for a cooling but
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Fig. 4. Backscattered electron images of representative ferromagnesian silicates from CPM and Scaup Lake flow rhyolites. A: Scaup Lake flow clinopyroxene containing core with
exsolved pigeonite, which is enlarged in panel B. Bright inclusions are magnetite. C: West Yellowstone flow clinopyroxene. D: West Yellowstone flow fayalite. E: Grants Pass flow
clinopyroxene. F: Pitchstone Plateau flow clinopyroxene. Scale bars are each 100 micrometers.
groundmass glass and phenocryst composition, as well as the general
homogeneity of individual clinopyroxenes and fayalite, suggests a
kinship between at least the rims of the ferromagnesian silicates and
their host melts. For example, clinopyroxene rims and fayalite from the
Solfatera Plateau flow correlate with the high Fe concentration of their
host glass. The overall temporal trend of clinopyroxene and fayalite
compositions to higher Fe/Mg values (Fig. 3), as well as the trend to
lower Ti concentrations in quartz, suggest crystallization of a magmatic
system that differentiated to more-evolved and cooler magma compositions. Similar compositional trends are observed for clinopyroxenes and
Fe-rich olivines in plutonic and volcanic rocks whose compositional
diversity was primarily generated by cooling and crystal-melt fractionation (e.g., Warshaw and Smith,1988; Morse, 1996). A temporal evolution
to more-evolved and cooler magma compositions is also supported by
the restriction of plagioclase and orthopyroxene to the oldest and least
evolved CPM rhyolites, as well as the replacement of orthopyroxene by
fayalite in the mineral assemblages of younger CPM rhyolites (Hildreth
et al., 1984).
Although the oldest CPM clinopyroxenes have compositions like
those in the Scaup Lake flow (Fig. 3), they are texturally distinct from
those in UBM rhyolites. CPM clinopyroxenes are generally homogenous and lack exsolution that is present in Scaup Lake flow
clinopyroxenes (Fig. 4). Exsolution in Fe-rich clinopyroxenes indicates
subsolidus unmixing in slowly cooled intrusions (e.g., Ashwal, 1982).
Accordingly, the cores of Scaup Lake flow pyroxenes are likely to be
recycled from a remelted intrusion. Zircons from the Scaup Lake flow
contain cores with relatively low δ18O values, suggesting that this lava
contains crystals recycled from a hydrothermally altered intrusion
(Bindeman et al., 2008).
The isotopic variation of CPM whole rocks and glasses rules out
closed-system fractionation and indicates that mixing and/or assimilation plays a role in CPM magma evolution (Doe et al., 1982; Hildreth
Please cite this article as: Vazquez, J.A., et al., Thermochemical evolution of young rhyolites at Yellowstone: Evidence for a cooling but
periodically replenished postcaldera..., Journal of Volcanology and Geothermal Research (2009), doi:10.1016/j.jvolgeores.2008.11.030
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7
Fig. 5. Cathodoluminescence images of quartz phenocrysts from CPM and Scaup Lake flows, arranged in general stratigraphic order from top left (youngest) to lower right (oldest).
Titanium concentrations (ppm) measured by ion microprobe are listed next to analysis locations (circles). Dark areas in cores of Grants Pass, Hayden Valley, and Scaup Lake flows are
glass inclusions. Scale bars are each 100 micrometers.
et al., 1984, 1991; Fig. 8). The evolution of CPM rhyolites to less
radiogenic Pb isotope values (Fig. 2), which is generally correlated
with higher 143Nd/144Nd compositions, suggests that new melts,
containing a greater proportion of juvenile Pb and Nd (Fig. 8),
recharged the subcaldera reservoir in a quasi-continuous fashion
between ~ 170 and 75 ka. Rather than evolving towards the low 143Nd/
144
Nd and high 206Pb/204Pb values associated with assimilation or
localized crustal melting at Yellowstone (e.g., UBM and extracaldera
rhyolites; Doe et al., 1982; Hildreth et al., 1991), the CPM melts evolve
over time towards the Nd and Pb isotope compositions characteristic
of Yellowstone basalts (Figs. 2 and 8). The low Sr and Ba concentrations rule out mixing between resident CPM rhyolite and mafic or
intermediate magmas. These characteristics suggest recharge by new
silicic magma from deeper, less contaminated levels of the magma
system, as proposed by Hildreth et al. (1991). Although direct evidence
for intrusion of basalt into the shallow rhyolitic reservoir, such as in
the form of rhyolite-hosted mafic inclusions, is not observed
(Christiansen, 2001), mafic intrusions trapped in the deeper portions
of the reservoir ultimately drive silicic magmatism at Yellowstone
(Hildreth et al., 1991; Christiansen, 2001; Lowenstern and Hurwitz,
2008). Nevertheless, assimilation of wallrock may still account for
some or all of the anomalous trace element and isotopic composition
of the Solfatara Plateau flow (discussed below).
5.2. Thermal evolution during differentiation of CPM rhyolites
The results from geothermometry indicate that CPM rhyolites record
crystallization over a large temperature interval. Discordance between
calculated temperatures using different thermometers might reflect real
differences in the temperatures of crystallization between phases (e.g.,
quartz vs. pyroxenes) or differences in the re-equilibration timescales
and/or calibrations associated with the geothermometers. The temperatures derived from ferromagnesian phenocryst pairs and quartz
phenocrysts are likely to reflect those of crystallization because
compositional re-equilibration of these minerals is sluggish at magmatic
temperatures (e.g., Brady, 1995; Cherniak et al., 2007). In contrast, Fe–Ti
oxide temperatures are likely to reflect near eruption conditions due to
their short timescales of re-equilibration (e.g., Nakamura, 1995). The
somewhat higher temperatures from Fe–Ti oxides for some lavas may
reflect heating immediately prior to eruption, and may account for the
Please cite this article as: Vazquez, J.A., et al., Thermochemical evolution of young rhyolites at Yellowstone: Evidence for a cooling but
periodically replenished postcaldera..., Journal of Volcanology and Geothermal Research (2009), doi:10.1016/j.jvolgeores.2008.11.030
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are significantly higher than their solidus and would correspond to
high melt proportions (Bindeman et al., 2008). In the repose intervals
between the different eruptive episodes, the CPM magma reservoir
might have rapidly fluctuated between near-liquidus and sub solidus
temperatures (Bindeman et al., 2008). However, eruptions of basaltic
lavas solely occurred outside caldera margins during the interval of
CPM volcanism, suggesting the presence of a persistent and laterally
extensive reservoir of suprasolidus rhyolite (Hildreth et al., 1984;
Christiansen, 2001).
5.3. Detailed thermal record in quartz phenocrysts
Fig. 6. Geothermometry CPM rhyolites and UBM Scaup Lake flow. A. Excluding the mixed
Solfatara Plateau flow and volumetrically minor Grants Pass flow, temperatures derived
from different thermometers for CPM rhyolites decrease over time. B. Temperatures for
Solfatara Plateau flow quartz are higher than those for coexisting ferromagnesian phases
as well as other CPM rhyolites (shaded field). Fe–Ti oxides from the Grants Pass flow yield
higher temperatures than coeval Pitchstone Plateau flow. Scaup Lake flow phenocrysts
yield a range of temperatures. Triangle for SCL is temperature from two-pyroxene
thermometry. Lavas abbreviations are as in Fig. 1.
apparent rounding (partial dissolution) of some fayalites and pyroxenes
(Fig. 4D and F). Those phenocrysts (Fig. 4C and E) with sharp interfacial
intersections suggest that they were stable immediately prior to
eruption. Despite any discordance, the different geothermometers
yield a similar sense of relative temperatures: rhyolite from the youngest
(~75 ka) volcanic episode yields the lowest temperatures and rhyolite
from the oldest (~ 170 ka) volcanic episode yields the highest
temperatures.
The CPM rhyolites yield some of the lowest temperatures for
rhyolites associated with the Yellowstone hotspot (cf. Perkins et al.,
1995; Nash et al., 2006). Temperatures for CPM rhyolites are mostly
lower than the 900°–925 °C range reported by Hildreth et al. (1984) for
UBM rhyolites, and the 850°–1000 °C range for older hotspot rhyolites
in the central and western Snake River Plain (Honjo et al., 1992, Perkins
et al., 1995; Cathey and Nash, 2004; Nash et al., 2006; Andrews et al.,
2007). High temperatures for SRP rhyolites are likely to reflect higher
rates of basaltic intrusion and crustal melting during the early history
of the Yellowstone hotspot (Bonnichsen et al., 2008). The ~150°
temperature range recorded by CPM lavas (Fig. 7) is comparable to the
100°–120° range documented for the compositionally zoned Lava
Creek and Huckleberry Ridge Tuffs (Hildreth, 1981; Hildreth et al.,
1984), which may be considered a “snapshot” of the thermal and
chemical gradients that exist within a caldera-forming rhyolite at
Yellowstone. The highest CPM temperatures (~ 900 °C) are near those
expected for liquidus conditions based on whole rock compositions
(Doe et al., 1982; Bindeman and Valley, 2001). Comparison of
calculated temperatures to solidus temperatures for granite with
bulk composition similar to CPM rhyolites (e.g., Clemens et al., 1986)
suggests that the youngest rhyolite (Pitchstone Plateau flow) crystallized at near-solidus temperatures (~750°–800°C) prior to eruption.
Nevertheless, most CPM phenocrysts crystallized at temperatures that
Quartz phenocrysts may retain a detailed and semi-continuous record
of thermal changes during crystallization due to the high sensitivity of the
TitaniQ thermometer (Wark and Watson, 2006). Relatively slow diffusion
of Ti in quartz at magmatic temperatures ensures that crystallization
temperatures are likely to be preserved by millimeter-size crystals
(Cherniak et al., 2007). Except for the Solfatara Plateau, temperatures for
CPM quartz are in general agreement with those from coexisting
ferromagnesian phases and mimic the trend to lower temperatures over
time (Fig. 6). Solfatara Plateau quartz have high Ti rims, suggesting an uptemperature evolution relative to their cores. In contrast, quartz
phenocrysts from other CPM rhyolites have relatively low Ti rims,
suggesting a down-temperature evolution relative to cores (Fig. 6). Like
the Solfatara Plateau flow, quartz phenocrysts from the UBM Scaup Lake
flow have relatively high Ti rims (Fig. 6).
Titanium variations and truncated zoning within single quartz
phenocrysts suggest an oscillation of magma temperatures over tens
of degrees during the general down-temperature evolution of CPM
rhyolites (Fig. 7). Irregular, truncated, and wavy boundaries between
zones identified by CL imaging suggest dissolution due to changes in
magma temperatures, composition, and/or decompression associated
with ascent of water-undersaturated rhyolite (Whitney, 1988; Shane
et al., 2008). The subhedral–anhedral shape of the quartz phenocrysts
(Fig. 5) suggests that one or a combination of these mechanisms
occurred immediately prior to eruption.
Fig. 7. CL image showing location of ion microprobe analyses (circles) and measured Ti
concentrations for West Yellowstone flow quartz phenocryst (YCV09-2-g10) mounted
in epoxy; lower right portion of crystal rim is bounded by groundmass glass (within
dashed line). Assuming constant aTiO2, temperatures from TitaniQ thermometry vary
by tens of degrees relative to the rim (ΔTrim), with the bright core yielding the highest
relative temperatures. Ti concentrations are in ppm; 1σ uncertainties are ± 1–2 ppm,
which equates to ±1–3 °C. Scale bar: 100 micrometers.
Please cite this article as: Vazquez, J.A., et al., Thermochemical evolution of young rhyolites at Yellowstone: Evidence for a cooling but
periodically replenished postcaldera..., Journal of Volcanology and Geothermal Research (2009), doi:10.1016/j.jvolgeores.2008.11.030
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9
Table 3
Representative compositions and results of geothermometry using ferromagnesian minerals and quartz
Lava flow and eruption age
Fayalite
XFo
XLa
Clinopyroxene
XEn
XWo
T (°C) cpx-fay
Fe–Ti oxides
XIlm
XUsp
T (°C) mt–il
logfO2
ΔFMQ
aTiO2 (mt–il)
T (°C) quartz
Cores
Rims
Pitchtone Plateau
79 ± 10 ka
Grants Pass
72 ± 3 ka
Solfatara Plateau
103 ± 8 ka
West Yellowstone
114 ± 1 ka
Dry Creek
166 ± 9 ka
Input
Calc
Input
Calc
Input
Calc
Input
Calc
Input
Calc
Input
Calc
0.051
0.005
0.056
0.005
–
–
–
–
0.023
0.008
0.028
0.008
0.106
0.007
0.131
0.007
–
–
–
–
–
–
–
–
0.100
0.368
0.100
0.372
755
–
–
–
–
–
0.049
0.372
0.049
0.372
810
0.178
0.376
0.178
0.376
845
–
–
–
–
–
0.352
0.409
0.352
0.409
⁎904 ± 29
0.900
0.323
0.926
0.531
806
−13.6
0.79
0.56
0.954
0.650
866
−13.3
−0.17
0.40
810 ± 17
778 ± 9
0.916
0.483
Scaup Lake
257 ± 13 ka
0.924
0.566
0.918
0.445
842
− 14.7
− 1.12
0.33
860
− 13.2
0.11
0.43
898
−12.8
−0.21
0.38
834
−13.6
0.17
0.45
880 ± 15
902 ± 11
844 ± 26
823 ± 12
889 ± 14
869 ± 15
849 ± 13
862 ± 10
Input values for clinopyroxene-fayalite temperatures are observed means of rims from phenocrysts. Fayalite compositions (italicized) are allowed to vary for equilibrium calculations
using QUILF-95 program (Anderson et al., 1993). Estimated uncertainties on clinopyroxene-fayalite and Fe–Ti oxide temperatures are approximately ±25°. ΔFMQ is log unit difference
from the fayalite–magnetite–quartz buffer. Activity of TiO2 is from magnetite–ilmenite equilibria (aTiO2 mt–il) using method described by Wark et al. (2007). Quartz temperatures
assume constant aTiO2 (mt–il) and are median values of core and rim analyses (± 1 s.d.). Eruption ages from Christiansen et al. (2007). ⁎QUILF temperature from clinopyroxene and
orthopyroxene rim compositions.
5.4. Solfatara Plateau rhyolite and late-stage addition of low δ18O
magma
The distinct compositions of Solfatara Plateau flow minerals and
glass suggest late-stage mixing with low δ18O magma. The Pb isotope
composition of Solfatara Plateau groundmass glass is significantly less
radiogenic than other CPM rhyolites (Fig. 2), with values that are
intermediate between UBM rhyolites and Yellowstone-area basalts
and/or Huckleberry Ridge Tuff (Fig. 8). Solfatara Plateau flow contains
zircons with low δ18O rims and groundmass glass with low δ18O
values, suggesting that the O-isotope composition of the melt was
lowered shortly before eruption (Bindeman et al., 2008). The CL-bright
and high Ti rims in Solfatara Plateau flow quartz may reflect this
mixing. If so, the high Ti rims suggest that mixing resulted in a heated
hybrid (e.g., Wark et al., 2007). Alternatively, mixing may have
lowered melt aTiO2, resulting in higher apparent temperatures for
quartz and discordance with other thermometers (Fig. 6). Other
Solfatara Plateau flow minerals suggest magma mixing. Sanidine
phenocrysts form a bimodal population with a subgroup that is
characterized by sieve-textured cores and another characterized by
oscillatory zoning. The distinct Nd–Pb isotopic composition of the
Solfatara Plateau flow effectively rules out wholesale remelting of
intrusions like the UBM lavas, precaldera rhyolites, or Lava Creek Tuff
(Fig. 8). Instead, the Nd–Pb–O-isotope composition might be generated by mixing CPM-like rhyolite with remelted Huckleberry Ridge
Tuff that has been hydrothermally altered and hybridized by intrusion
of new rhyolites. However, dating of Solfatara Plateau zircons by
Vazquez and Reid (2002) did not reveal Huckleberry Ridge Tuff-aged
xenocrysts, suggesting that zircons inherited from the tuff, if present,
were completely resorbed.
5.5. Compositional zoning of the voluminous and final CPM eruption
The youngest (ca. 75 ka) episode of CPM volcanism produced two
lavas, the Grants Pass and Pitchstone Plateau flows, from closely spaced
vents along the western caldera ring-fracture (Christiansen et al.,
2007). The Grant Pass flow is only a small proportion (≤1%) of the
episode's eruptive volume (~ 70 km3), and may represent the initial
effusion of a rhyolite dike prior to central vent eruption of the
voluminous Pitchstone Plateau flow (Christiansen et al., 2007). Relative
to the Pitchstone Plateau flow, the Grants Pass flow has less-evolved
glass (Fig. 2), clinopyroxene with lower Fe/Mg (Table 2, Fig. 3), higher
Fe–Ti oxide temperatures (Fig. 6), and a more radiogenic Pb isotopic
composition (Fig. 2). Fayalite is absent, or rare, in the Grants Pass flow
yet abundant in the Pitchstone Plateau flow. These differences, as well
as the close temporal and spatial association of both rhyolites, suggest
that the youngest volcanic episode tapped a body of magma zoned in
composition and temperature or stored in separate chambers. If from a
single body of magma, then the eruption is analogous to those
explosive eruptions that result in reversely zoned ignimbrites (e.g.,
Hildreth, 1981) and some silicic lavas (e.g., Duffield and Ruiz, 1992).
Alternatively, the Grants Pass–Pitchstone Plateau eruption may have
tapped closely spaced yet isolated lenses of melt-rich rhyolite within a
larger reservoir of near-solidus magma (e.g., Charlier et al., 2003;
Hildreth, 2004). Indeed, the differences in composition between coeval
rhyolites from the same eruptive groups might reflect storage in a
plexus of melt lenses that evolved in tandem.
5.6. Evolving composition of CPM rhyolites: cooling-induced fractionation
or serial remelting?
Two general models for the diversity and evolution of CPM rhyolites
have been proposed: 1) fractionation and hybridization of a voluminous
magma reservoir (Hildreth et al., 1984, 1991; Christiansen, 2001; Vazquez
and Reid, 2002) and 2) localized wholesale remelting–recycling of shallow
and altered wallrocks and caldera fill by intrusions of basaltic magma
(Bindeman and Valley, 2001; Bindeman et al., 2008). The results from
geothermometry provide important constraints on these models, and may
provide insight into the evolution of composition and time groups at older
calderas of the hotspot. The former model is primarily based on the agecorrelated isotopic and geochemical characteristics of Yellowstone
rhyolites, whereas the latter model is primarily based on the oxygen
isotope heterogeneity of phenocrysts in UBM lavas. Using the oxygen
isotope compositions of the Solfatara Plateau and Scaup Lake flows as
representatives of CPM rhyolites, Bindeman et al. (2008) suggest that CPM
rhyolites result from remelting of UBM or Lava Creek Tuff wallrocks.
Please cite this article as: Vazquez, J.A., et al., Thermochemical evolution of young rhyolites at Yellowstone: Evidence for a cooling but
periodically replenished postcaldera..., Journal of Volcanology and Geothermal Research (2009), doi:10.1016/j.jvolgeores.2008.11.030
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J.A. Vazquez et al. / Journal of Volcanology and Geothermal Research xxx (2009) xxx–xxx
such as the Lava Creek Tuff and UBM rhyolites. In order to generate the
observed age-correlated compositional and isotopic trends, the locus of
subcaldera anatexis would need to follow a sequence of remelting in
which more-fractionated yet more-juvenile intrusions were targeted
over time, with a net result of generally cooler magmas over time. We
consider this sequence of melting and magma evolution to be unlikely.
Instead, the age-correlated trends to more-evolved, more-juvenile, and
near-solidus rhyolite compositions are better reconciled by a magma
reservoir undergoing differentiation via fractional crystallization,
recharge, and assimilation, as originally proposed by Hildreth et al.
(1984, 1991). In addition, the results suggest that CPM rhyolites
represent a significant rejuvenation of the shallow subcaldera reservoir.
Christiansen (2001) concluded that the field relations and distinct
petrologic characteristics of CPM rhyolites reflect ascent of voluminous
magma from deeper levels of the system, which reheated and caused a
“resetting” of the congealed portions of the reservoir responsible for the
UBM and Lava Creek Tuff eruptions. The elevated temperatures of the
oldest CPM rhyolites and their distinct petrography relative to the Scaup
Lake flow also support a model for episodic reintrusion and rejuvenation
of the shallow reservoir.
6. Conclusions
Fig. 8. Lead–neodymium isotope composition of CPM and Scaup Lake flow glasses (circles).
SCL: Scaup Lake flow, SP: Solfatara Plateau flow, LCT: Lava Creek Tuff, UBM: Upper Basin
Member rhyolites, HRT: Huckleberry Ridge Tuff, PC: Precaldera rhyolite (Lewis Canyon flow),
YB: Yellowstone Plateau basalts, EXT: extracaldera rhyolites. Fields are from Doe et al. (1982)
and Hildreth et al. (1984, 1991). Pb values normalized to NBS 981 standard for comparison.
However, our results demonstrate that these two lavas are anomalous
rather than representative of CPM rhyolites. Solfatara Plateau flow
phenocrysts and glass are compositionally and isotopically distinct
(Figs. 2, 3, 6, 8) relative to other CPM flows, suggesting a relatively unique
evolution. In contrast to CPM rhyolites, the Scaup Lake flow contains
phenocrysts with abundant disequilibrium textures and evidence for
inheritance of major phases (Fig. 5). In addition, the ~260 ka Scaup Lake
flow erupted about 100 ka before the oldest CPM lava (Christiansen et al.,
2007), as well as tens of thousands of years before crystallization of
“autocrystic” (Miller et al., 2007) zircons in the oldest CPM rhyolites
(Vazquez and Reid, 2002), and is part of a different (UBM) member
(Christiansen and Blank, 1972).
The compositional and petrographic differences between CPM and
UBM minerals and glasses are important for evaluating the roles of
fractionation and remelting at Yellowstone. In general, CPM clinopyroxenes and fayalites are homogeneous, yet compositionally distinct
between eruptive episodes. This observation suggests that most CPM
phenocrysts are indigenous to their host magmas with little recycling
of crystals from one volcanic episode to the next. In contrast, the Scaup
Lake flow, as well as older UBM rhyolites, contains exsolved pyroxene,
suggesting remelting and recycling of subsolidus intrusions (cf. Bacon
and Lowenstern, 2005). Inspection of pyroxenes from the ca. 500 ka
Middle Biscuit Basin flow (Gansecki et al., 1996) that likely results
from remelting of subsolidus intrusions based on its low δ18O values
(Bindeman and Valley, 2001; Bindeman et al., 2008), reveals the
same type of blebby and lamellar exsolution as in Scaup Lake flow
pyroxenes. The age-compositional trends of CPM rhyolites would be
difficult to generate simply by serial remelting of shallow caldera rocks
The compositions of ferromagnesian phenocrysts and groundmass
glasses from lavas composing the postcaldera Central Plateau Member
rhyolites at Yellowstone caldera are generally correlated with eruption
age and suggest a temporal evolution to more-evolved and cooler
compositions of high-silica rhyolite. Application of multiple
geothermometers reveals crystallization temperatures that span an
approximately 150° interval between liquidus and solidus conditions
as well as a temporal trend from high to low temperatures. Ti zoning
within single quartz phenocrysts indicate that evolution to lower
temperatures was punctuated by fluctuations of tens of degrees.
Isotopic variability of erupted melts indicates open-system evolution,
likely due to input of more-juvenile rhyolites and assimilation of
young wallrocks. These characteristics are consistent with a voluminous magma reservoir undergoing cooling-induced fractionation,
recharge, and assimilation. Comparison of CPM and UBM rhyolites
reveals significant petrographic and compositional differences, suggesting that wholesale remelting of intrusions was most important for
the oldest group of postcaldera rhyolites.
Acknowledgements
We thank Rick Hervig, Jeremy Boyce, Linda Williams, Frank Kyte, and
Frank Tepley for assistance in using various microprobe instruments.
Axel Schmitt kindly provided a spreadsheet for calculation of aTiO2 from
Fe–Ti oxides. We are grateful to Barbara Nash and Charles Bacon for their
excellent and careful reviews. This research was supported by NSF
awards EAR-0538113, EAR-0538309, and GEO.-0538113.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jvolgeores.2008.11.030.
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